Most new aromatics complexes are designed to maximize the yield of benzene and para-xylene. Benzene is a versatile petrochemical building block used in many different products based on its derivation including ethylbenzene, cumene, and cyclohexane. Para-xylene is also an important building block, which is used almost as exclusively for the production of polyester fibers, resins, and films formed via terephthalic acid or dimethyl terephthalate intermediates. Accordingly, an aromatics complex may be configured in many different ways depending on the desired products, available feedstocks, and investment capital available. A wide range of options permits flexibility in varying the product slate balance of benzene and para-xylene to meet downstream processing requirements.
A prior art aromatics complex flow scheme has been disclosed by Meyers in part 2 of the Handbook of Petroleum Refining Processes, 2d. Edition, in 1997 published by McGraw-Hill.
In general, a xylene production facility can have various types of processing reactions. One is a transalkylation in which benzene and/or toluene are reacted with C9+ aromatics to form more methylated aromatics. Another is xylene isomerization, which may also include dealkylation, where a non-equilibrium mixture of xylenes is isomerized. The ethylbenzene may be isomerized to xylenes or may be dealkylated to yield, e.g., benzene. And another is disproportionation in which toluene is disproportionated. The disproportionation reaction yields one mole of benzene per mole of xylene produced.
The benzene sought for other chemical processes and for commercial sale is often required to meet stringent purity standards. One major use of benzene is as a feed to make cyclohexane which in turn can be used to make nylons. For this use, the benzene purity demanded is at least about 99.85 mass percent. Other uses of benzene may have different purity demands, but usually the purity must be at least about 99.0, most commonly at least about 99.5, mass percent.
Accordingly, for a xylene production facility to be a viable source of benzene, it must be able to achieve benzene purity. However, the feeds to xylene production units are generally derived from petroleum reforming and contain many impurities including C6 and C7 non-aromatics, including cyclic aliphatics, that are difficult to remove from benzene by distillation and are often referred to as benzene co-boilers. Additionally, benzene co-boilers may be by-products of catalytic unit operations used to make xylenes from other aromatics contained in the various streams in a xylene production facility. Moreover, impurities in the feeds may be deleterious to catalysts used in these unit operations.
Commonly the feed or a fraction of the feed containing benzene and toluene has been subjected to an extraction process to remove non-aromatic components, including benzene co-boilers. The extraction removes components that could adversely affect catalysts in various catalytic unit operations in the facility. And, because the benzene co-boilers have been removed, a benzene stream from the disproportionation can be recovered as a highly pure stream through distillation.
U.S. Pat. No. 6,740,788 to Maher, et al., disclose a process in which the feed to a transalkylation reactor is fractionated in a benzene column prior to being passed to the reactor. The overhead from the column is extracted to remove non-aromatics in a raffinate and highly pure benzene is obtained. The patentees state that the entire feed to the transalkylation reactor need not be subjected to such an extraction process provided that the transalkylation catalyst has been stabilized through the introduction of a metal function.
U.S. Pat. Publ. No. 2004/0186330 of Kong, et al., discloses a process for producing xylenes containing a transalkylation section, a disproportionation section and an isomerization section. The disclosure provides for the extraction of non-aromatics from the benzene and toluene-containing portion of the feed. No disclosure is provided about obtaining a benzene product. Benzene from the disproportionation is recycled to the transalkylation section to make toluene.
U.S. Pat. Publ. No. 2004/0186332 of Kong, et al., discloses a process for producing xylenes using a disproportionation and transalkylation of toluene and heavy aromatics.
U.S. Pat. No. 4,341,914 to Berger discloses a transalkylation process with recycle of C10 alkylaromatics in order to increase yield of xylenes from the process. The transalkylation process is also preferably integrated with a para-xylene separation zone and a xylene isomerization zone operated as a continuous loop receiving mixed xylenes form the transalkylation zone feedstock and effluent fractionation zones.
Integrated processes are desired that maximize the production of xylenes while still enabling high purity benzene to be provided as a product stream without the requirement for an extraction unit operation.